1. Probiotic Microorganisms
The gastrointestinal microflora has been shown to play a number of vital roles in maintaining gastrointestinal tract function and overall physiological health. For example, the growth and metabolism of the many individual bacterial species inhabiting the gastrointestinal tract depend primarily upon the substrates available to them, most of which are derived from the diet. See e.g., Gibson G. R. et al., 1995. Gastroenterology 106: 975-982; Christi, S. U. et al., 1992. Gut 33: 1234-1238. These finding have led to attempts to modify the structure and metabolic activities of the community through diet, primarily with probiotics which are live microbial food supplements. The best known probiotics are the lactic acid-producing bacteria (i.e., Lactobacilli) and Bifidobacteria, which are widely utilized in yogurts and other dairy products. These probiotic organisms are non-pathogenic and non-toxigenic, retain viability during storage, and survive passage through the stomach and small intestine. Since probiotics do not permanently colonize the host, they need to be ingested regularly for any health promoting properties to persist. Commercial probiotic preparations are generally comprised of mixtures of Lactobacilli and Bifidobacteria, although yeast such as Saccharomyces have also been utilized.
Probiotic preparations were initially systematically evaluated for their effect on health and longevity in the early-1900's (see e.g., Metchinikoff, E., Prolongation of Life, Willaim Heinermann, London 1910), although their utilization has been markedly limited since the advent of antibiotics in the 1950's to treat pathological microbes. See e.g., Winberg, et al, 1993. Pediatr. Nephrol. 7: 509-514; Malin et al, Ann. Nutr. Metab. 40: 137-145; and U.S. Pat. No. 5,176,911. Similarly, lactic acid-producing bacteria (e.g., Bacillus, Lactobacillus and Streptococcus species) have been utilized as food additives and there have been some claims that they provide nutritional and/or therapeutic value. See e.g., Gorbach, 1990. Ann. Med. 22: 37-41; Reid et al, 1990. Clin. Microbiol. Rev. 3: 335-344.
Therefore, probiotic microorganisms are those which confer a benefit when grow in a particular environment, often by inhibiting the growth of other biological organisms in the same environment. Examples of probiotic organisms include bacteria and bacteriophages which possess the ability to grow within the gastrointestinal tract, at least temporarily, to displace or destroy pathogenic organisms, as well as providing other benefits to the host. See e.g., Salminen et al, 1996. Antonie Van Leeuwenhoek 70: 347-358; Elmer et al, 1996. JAMA 275: 870-876; Rafter, 1995. Scand. J. Gastroenterol. 30: 497-502; Perdigon et al, 1995. J. Dairy Sci. 78: 1597-1606; Gandi, Townsend Lett. Doctors & Patients, pp. 108-110, January 1994; Lidbeck et al, 1992. Eur. J. Cancer Prev. 1:341-353.
The majority of previous studies on probiosis have been observational rather than mechanistic in nature, and thus the processes responsible for many probiotic phenomena have yet to be quantitatively elucidated. Some probiotics are members of the normal colonic microflora and are not viewed as being overtly pathogenic. However, these organisms have occasionally caused infections (e.g., bacteremia) in individuals who are, for example, immunocompromised. See e.g., Sussman, J. et al., 1986. Rev Infect. Dis. 8: 771-776; Hata, D. et al., 1988. Pediatr. Infect. Dis. 7: 669-671.
While the attachment of probiotics to the gastrointestinal epithelium is an important determinant of their ability to modify host immune reactivity, this is not a universal property of Lactobacilli or Bifidobacteria, nor is it essential for successful probiosis. See e.g., Fuller, R., 1989. J. Appl. Bacteriol. 66: 365-378. For example, adherence of Lactobacillus acidophilus and some Bifidobacteria to human enterocyte-like CACO-2 cells has been demonstrated to prevent binding of enterotoxigenic and enteropathogenic Escherichia coli, as well as Salmonella typhimurium and Yersinia pseudotuberculosis. See e.g., Bernet, M. F. et al., 1994. Gut 35: 483-489; Bernet, M. F. et al., 1993. Appl. Environ. Microbiol. 59: 4121-4128.
While the gastrointestinal microflora presents a microbial-based barrier to invading organisms, pathogens often become established when the integrity of the microbiota is impaired through stress, illness, antibiotic treatment, changes in diet, or physiological alterations within the G.I. tract. For example, Bifidobacteria are known to be involved in resisting the colonization of pathogens in the large intestine. See e.g., Yamazaki, S. et al., 1982. Bifidobacteria and Microflora 1: 55-60. Similarly, the administration of Bifidobacteria breve to children with gastroenteritis eradicated the causative pathogenic bacteria (i.e., Campylobacter jejuni) from their stools (see e.g., Tojo, M., 1987. Acta Pediatr. Jpn. 29: 160-167) and supplementation of infant formula milk with Bifidobacteria bifidum and Streptococcus thermophilus was found to reduce rotavirus shedding and episodes of diarrhea in children who were hospitalized (see e.g., Saavedra, J. M., 1994. The Lancet 344: 1046-109.
In addition, some lactic acid producing bacteria also produce bacteriocins which are inhibitory metabolites which are responsible for the bacteria's anti-microbial effects. See e.g., Klaenhammer, 1993. FEMS Microbiol. Rev. 12: 39-85; Barefoot et al., 1993. J. Diary Sci. 76: 2366-2379. For example, selected Lactobacillus strains which produce antibiotics have been demonstrated as effective for the treatment of infections, sinusitis, hemorrhoids, dental inflammations, and various other inflammatory conditions. See e.g., U.S. Pat. No. 5,439,995. Additionally, Lactobacillus reuteri has been shown to produce antibiotics which possess anti-microbial activity against Gram negative and Gram positive bacteria, yeast, and various protozoan. See e.g., U.S. Pat. Nos. 5,413,960 and 5,439,678.
Probiotics have also been shown to possess anti-mutagenic properties. For example, Gram positive and Gram negative bacteria have been demonstrated to bind mutagenic pyrolysates which are produced during cooking at a high temperature. Studies performed with lactic acid-producing bacteria has shown that these bacteria may be either living or dead, due to the fact that the process occurs by adsorption of mutagenic pyrolysates to the carbohydrate polymers present in the bacterial cell wall. See e.g., Zang, X. B. et al., 1990. J. Dairy Sci. 73: 2702-2710. Lactobacilli have also been shown to degrade carcinogens (e.g., N-nitrosamines), which may serve an important role if the process is subsequently found to occur at the level of the mucosal surface. See e.g., Rowland, I. R. and Grasso, P., Appl. Microbiol. 29: 7-12. Additionally, the co-administration of lactulose and Bifidobacteria longum to rats injected with the carcinogen azoxymethane was demonstrated to reduce intestinal aberrant crypt foci, which are generally considered to be pre-neoplastic markers. See e.g., Challa, A. et al., 1997. Carcinogenesis 18: 5175-21. Purified cell walls of Bifidobacteria may also possess anti-tumorigenic activities in that the cell wall of Bifidobacteria infantis induces the activation of phagocytes to destroy growing tumor cells. See e.g., Sekine, K. et al., 1994. Bifidobacteria and Microflora 13: 65-77. Bifidobacteria probiotics have also been shown to reduce colon carcinogenesis induced by 1,2-dimethylhydrazine in mice when concomitantly administered with fructo-oligosaccharides (FOS; see e.g., Koo, M. B., and Rao, A. V., 1991. Nutrit. Rev. 51: 137-146), as well as inhibiting liver and mammary tumors in rats (see e.g., Reddy, B. S., and Rivenson, A., 1993. Cancer Res. 53: 3914-3918).
It has also been demonstrated that the microbiota of the gastrointestinal tract affects both mucosal and systemic immunity within the host. See e.g., Famularo, G. et al., Stimulation of Immunity by Probiotics. In: Probiotics: Therapeutic and Other Beneficial Effects. pg. 133-161. (Fuller, R., ed. Chapman and Hall, 1997). The intestinal epithelial cells, blood leukocytes,
B- and T-lymphocytes, and accessory cells of the immune system have all been implicated in the aforementioned immunity. See e.g., Schiffrin, E. J. et al., 1997. Am. J. Clin. Nutr. 66(suppl): 5-20S. Other bacterial metabolic products which possess immunomodulatory properties include: endotoxic lipopolysaccharide, peptidoglycans, and lipoteichoic acids. See e.g., Standiford, Tex., 1994. Infect. Linmun. 62: 119-125. Accordingly, probiotic organisms are thought to interact with the immune system at many levels including, but not limited to: cytokine production, mononuclear cell proliferation, macrophage phagocytosis and killing, modulation of autoimmunity, immunity to bacterial and protozoan pathogens, and the like. See e.g., Matsumara, K. et al., 1992. Animal Sci. Technol. (Jpn) 63: 1157-1159; Solis-Pereyra, B. and Lemmonier, D., 1993. Nutr. Res. 13: 1127-1140. Lactobacillus strains have also been found to markedly effect changes in inflammatory and immunological responses including, but not limited to, a reduction in colonic inflammatory infiltration without eliciting a similar reduction in the numbers of B- and T-lymphocytes. See e.g., De Simone, C. et al., 1992. Immunopharmacol. Immunotoxicol. 14: 331-340.
2. Gastrointestinal Effects of Antibiotic Administration
Antibiotics are widely used to control pathogenic microorganisms in both humans and animals. Unfortunately, the widespread use of anti-microbial agents, especially broad spectrum antibiotics, has resulted in a number of serious clinical consequences. For example, antibiotics often kill beneficial, non-pathogenic microorganisms (i.e., flora) within the gastrointestinal tract which contribute to digestive function and health. Accordingly, relapse (the return of infections and their associated symptoms) and secondary opportunistic infections often result from the depletion of lactic acid-producing and other beneficial flora within the gastrointestinal tract.
Unfortunately, most, if not all, lactic acid-producing or probiotic bacteria are extremely sensitive to common antibiotic compounds. Accordingly, during a normal course of antibiotic therapy, many individuals develop a number of deleterious physiological side-effects including: diarrhea, intestinal cramping, and sometimes constipation. These side-effects are primarily due to the non-selective action of antibiotics, as antibiotics do not possess the ability to discriminate between beneficial, non-pathogenic and pathogenic bacteria, both bacterial types are killed by these agents. Thus, individuals taking antibiotics offer suffer from gastrointestinal problems as a result of the beneficial microorganisms (i.e., intestinal flora), which normally colonize the gastrointestinal tract, being killed or severely attenuated. The resulting change in the composition of the intestinal flora can result in vitamin deficiencies when the vitamin-producing intestinal bacteria are killed, diarrhea and dehydration and, more seriously, illness should a pathogenic organism overgrow and replace the remaining beneficial gastrointestinal bacteria.
Another deleterious result of indiscriminate use of anti-microbial agents is the generation of multiple antibiotic-resistant pathogens. See e.g., Mitchell, P. 1998. The Lancet 352: 462-463; Shannon, K., 1998. Lancet 352: 490-491. The initial reports of meticillin-resistant Staphylococcus aurous (MRSA) infections have been over-shadowed by the more recent outbreaks of vancomycin-resistant Enterococci (VRE). The development of such resistance has led to numerous reports of systemic infections which remained untreatable with conventional antibiotic therapies. Recently, a vancomycin—(generally regarded as an antibiotic of “last resort”) resistant strain of Staphylococcus aurous was responsible for over 50 deaths in a single Australian hospital. See e.g., Shannon, K., 1998. Lancet 352: 490-491.
Enterococci are currently a major nosocomial pathogen and are likely to remain as such for a long period of time. Enterococci, as well as other microbes, obtain antibiotic resistance genes in several different ways. For example, Enterococci emit pheromones which cause them to become “sticky” and aggregate, thus facilitating the exchange of genetic material, such as plasmids (autonomously replicating, circular DNA which often carry the antibiotic resistance genes). In addition, some Enterococci also possess “conjugative transposons” which are DNA sequences that allow them to directly transfer resistance genes without plasmid intermediary. It is believed that penicillin resistance has been conferred from Enterococci to Streptococci to Staphylococci through this later mechanism.
Since 1989, a rapid increase in the incidence of infection and colonization with vancomycin-resistant Enterococci (VRE) has been reported by numerous hospitals within the United States. This increase poses significant problems, including: (i) the lack of available anti-microbial therapy for VRE infections, due to the fact that most VRE are also resistant to the drugs which were previously used to treat such infections (e.g., Aminoglycosides and Ampicillin); and (ii) the possibility that the vancomycin-resistant genes present in VRE can be transferred to other gram-positive microorganisms (e.g., Staphylococcus aureus).
An increased risk for VRE infection and colonization has also been associated with previous vancomycin and/or multi-anti-microbial therapy, severe underlying disease or immunosuppression, and intra-abdominal surgery. Because Enterococci can be found within the normal gastrointestinal and female genital tracts, most enterococcal infections have been attributed to endogenous sources within the individual patient. However, recent reports of outbreaks and endemic infections caused by Enterococci, including VRE, have indicated that patient-to-patient transmission of the microorganisms can occur through either direct contact or through indirect contact via (i) the hands of personnel; or (ii) contaminated patient-care equipment or environmental surfaces.
Accordingly, there remains a need for a highly efficacious biorational therapy which functions to mitigate the deleterious physiological effects of digestive pathogens, including antibiotic-resistant gastrointestinal tract pathogens, in both humans and animals, by the colonization (or re-colonization) of the gastrointestinal tract with probiotic microorganisms, following the administration of antibiotics, anti-fungal, anti-viral, and similar agents. Additionally, a need as remains for the development of a highly efficacious biorational therapy which functions to mitigate antibiotic-resistant digestive pathogens, in both humans and animals, by the colonization (or re-colonization) of the gastrointestinal tract with probiotic microorganisms, following the administration of antibiotics, anti-fungal, anti-viral, and similar agents, by functioning to reduce both the colonization rate and the potential physiologically deleterious effects due to the colonization of antibiotic-resistant digestive pathogens.